Digital data (packet switching) communication networks have conventionally employed dedicated terrestrial circuits, such as landline telephone systems, to connect a host (mainframe) computer, located at a central or master station, with a plurality of geographically dispersed remote terminals, the locations of which are selected in an effort to meet current and projected communication demands of the system user. Because a dedicated landline telephone link in a multidrop network is an effectively rigid physical communication highway and typically employs some form of master-to-remote polling (point-to-point) mechanism for controlling communications between the master station and the remote stations, both the number and the locations of the stations of the network must be carefully chosen. In addition, it is common practice in packet-switched landline transmission networks to use point-to-point communication protocols between the user terminal and a network entry node, which require dedicated channel connections between the communication ports of the packet switches of the respective stations.
A satellite communication network, on the other hand, offers the user significant flexibility in the deployment of the stations, but is normally does not allow the use of a polling mechanism for controlling access to the communication channel, as in the case of a terrestrial system, because of the substantial transmission delay (wait) penalty that would be incurred. Consequently, a satellite communications network may preferably employ a communication channel that is accessed on a contention or demand assignment basis by the stations, in order to afford maximum, efficient utilization. In such a network, communications from the master station to the remote stations (outlink transmissions) are broadcast over a first, continuously transmitted frequency (channel) that is monitored by each remote station for messages addressed to it. Messages from the remote stations to the master station (return link transmissions) are transmitted over a second, shared channel, in burst mode, contention format.
Because of the manner in which the satellite communications channels are shared among a plurality of stations, they cannot be readily interfaced with terminal equipment (packet assembly/disassembly circuits for coupling the satellite network to existing landline connection ports). Namely, the local packet interface equipment may typically employ a point-to-point communication protocol, such as X.25 communication protocol, the station-to-station control layer of which contains a transmit/receive channel designation field and implies point-to-point utilization, exclusively. In order for such a protocol to be usable in a multistation satellite network, each earth station (master or remotes) would require a separate channel and port dedicated to each terminal being serviced, something that is practically impossible to achieve in a system that may serve thousands of terminal devices and, because of its use of a shared communications channel, effectively appears as a multidrop network, which is inherently incompatible with point-to-point communication protocols.
An additional problem that is encountered in the use of a shared (contention) communications network is the need for a collision/avoidance mechanism on the shared (remote-to-master) link. Namely, although outlink messages from the master station to the remote stations originate at only a single source (the master station), so that the issue of master-to-remote transmission collisions does not exist, remote stations transmit over the return link channel on a contention basis, so that there is the possibility for remote-to-master transmission collisions.
Efforts to reduce the collision problem in networks employing shared communication channels have included a variety of "permission"-based communication protocols, such as polling mechanisms (intolerable in a satellite network, as noted previously) and time division multiple access transmission formats, which operate, in effect, like polling mechanisms. In a commercial environment, where every effort is made to optimize channel occupancy and throughput, the delay penalty of such protocols makes them unacceptable candidates for handling traffic that may originate from literally thousands of system users (terminal devices) that are served by the stations of some networks.
Unfortunately, conventional collision avoidance/recovery schemes (such as that used in a slotted Aloha communication control mechanism) are effectively unworkable for the class of earth stations known as VSATs (very small aperture terminals) due to the fact that the transmitting (remote) stations are unable to monitor their own signals, because of the VSAT's small antenna and low transmit power. Instead, they rely on the transmission of acknowledgements from the master station to confirm message throughput. Similarly, master-to-remote messages are acknowledged by the remote stations.
Because an acknowledgement is essentially overhead, in terms of message size, it's length is small (on the order of ten bytes or less) compared with the length of a normal data packet (on the order of a thousand bytes). As a consequence, its impact on channel efficiency is particularly noticeable when this or other type of reduced content overhead messages is transmitted as a `partially-filled` data packet during a normal, fixed data time slot, the remaining unused portion of which may occupy a considerable percentage of the available transmission interval.
A further difficulty that is encountered in demand assignment, burst mode transmission schemes is the substantial reduction in network throughput that occurs when incoming (to be transmitted) traffic at remote stations build up to a level that effectively overloads the network, or reaches the onset of a saturation condition, so as to substantially increase transmission delay to the point that nearly every packet must be retransmitted, due to collisions with other bursts. As a result, the likelihood of a message from a remote station successfully reaching the master station is infinitesimally small, thus reducing network throughput to zero.